Abstracts

Borehole instabilities arefrequently encountered when drilling through finely laminated,organic rich shales (kland and Cook, 1998; Ottesen, 2010; etc.);such instabilities should be avoided to assure a successfulexploitation and safe production of the contained unconventionalhydrocarbons. Borehole instabilities, such as borehole breakouts ordrilling induced tensile fractures, may lead to poor cementing ofthe borehole annulus, difficulties with recording andinterpretation of geophysical logs, low directional control and inthe worst case the loss of the well. If these problems are notrecognized and expertly remedied, pollution of the groundwater orthe emission of gases into the atmosphere can occur since themigration paths of the hydrocarbons in the subsurface are not yetfully understood (e.g., Davies et al., 2014; Zoback et al., 2010).In addition, it is often mentioned that the drilling problemsencountered and the resulting downtimes of the wellbore system infinely laminated shales significantly increase drilling costs(Fjaer et al., 2008; Aadnoy and Ong, 2003). In order tounderstand and reduce the borehole instabilities during drilling inunconventional shales, we investigate stress-induced irregularextensions of the borehole diameter, which are also referred to asborehole breakouts. For this purpose, experiments with differentborehole diameters, bedding plane angles and stress boundaryconditions were performed on finely laminated Posidonia shales. TheLower Jurassic Posidonia shale is one of the most productive sourcerocks for conventional reservoirs in Europe and has the greatestpotential for unconventional oil and gas in Europe (Littke et al.,2011). In this work, Posidonia shale specimens from the North(PN) and South (PS) German basins were selected and characterizedpetrophysically and mechanically. The composition of the two shalesis dominated by calcite (47-56%) followed by clays (23-28%) andquartz (16-17%). The remaining components are mainly pyrite andorganic matter. The porosity of the shales varies considerably andis up to 10% for PS and 1% for PN, which is due to a largerdeposition depth of PN. Both shales show marked elasticity andstrength anisotropy, which can be attributed to a macroscopicdistribution and orientation of soft and hard minerals. Under loadthe hard minerals form a load-bearing, supporting structure, whilethe soft minerals compensate the deformation. Therefore, if loadedparallel to the bedding, the Posidonia shale is more brittle thanloaded normal to the bedding. The resulting elastic anisotropy,which can be defined by the ratio of the modulus of elasticityparallel and normal to the bedding, is about 50%, while thestrength anisotropy (i.e., the ratio of uniaxial compressivestrength normal and parallel to the bedding) is up to 66%. Based onthe petrophysical characterization of the two rocks, a transverseisotropy (TVI) was derived. In general, PS is softer and weakerthan PN, which is due to the stronger compaction of the materialdue to theAdvisors/Committee Members: Dresen, Georg (advisor).